Commonly, in a large-scale inkjet printer, ink is supplied to an inkjet head from an ink cartridge which is detachably mounted. Some of the inks such as metallic ink, pearl ink, white ink and the like contain fine particles (pigment or the like) whose specific gravity is different from liquid component. The specific gravity of the fine particle which is contained in the ink is large in comparison with that of the liquid component and the fine particle is, for example, made of metal or ore.
When the ink is left to stand for a long time under an environment that ink flow is stopped, fine particles whose specific gravity is large are precipitated down in the liquid and, as a result, clogging of piping and failure of ejection may be occurred.
Further, a cross-sectional area and a volume of piping are changed due to installation of a joint or a sub tank based on arranging layout of piping and functions of an inkjet printer. Stagnation of ink may occur in the portion when a used amount of the ink is small and, as a result, the fine particles are precipitated to cause a malfunction of the printer and thus a desired printed object is not obtained.
Further, in the inkjet printer, at the time of introducing ink or the like, bubbles stagnated in the middle of piping or in a common ink flow passage of the head are carried to a nozzle together with the ink, which may cause a failure of ejection.
A method for circulating ink may be used in order to solve the problem. For precipitation, ink is always moved through circulation of the ink and thereby the precipitation is prevented by agitating action through the flow. Further, for the bubble, the stagnated bubbles are flowed to a bubble trap or an ink reservoir tank to eliminate the bubbles.
The circulation provides the above-mentioned merits but attention should be given to a pressure control. A pressure at a nozzle portion in an inkjet head gives a large effect to the ejection and thus an ink pressure at the nozzle portion is controlled at a fixed negative pressure and thereby a meniscus having a predetermined shape is formed in the nozzle.
Therefore, conventionally, ink is circulated while adjusting the pressure so as not to affect the meniscus formed in each nozzle (see, for example, Patent Literature 1).